1. Field of the Invention
This invention relates generally to video display monitors and more particularly to a method and apparatus for reducing visibility of damping wires in aperture grill display tubes.
2. Description of the Background Art
Accurate representation of visual information is a significant consideration of manufacturers, designers and user of video display monitors. Aperture grill cathode ray tubes (CRTs) are devices which are often used in conventional video display monitors.
Referring now to FIG. 1, a diagram of a display CRT 110 is shown, according to the present invention. CRT 110 includes a glass screen 112 with an inner surface that is covered with a phosphor coating 114. In aperture grill CRTs, an electron gun 116 causes electron beams 118 to pass through vertical slits etched in a thin sheet of metal which comprises an aperture grill 120. The slits are part of the mechanism that directs the red, green and blue electron beams 118 to their respective phosphors 114. The electron beams 118 emitted from the electron gun 116 assembly pass through the aperture grill 120 on the way to the phosphor coating 114, which responds to the electron bombardment by emitting light. The aperture grill 120 partially blocks the electron beams 118, casting shadows on the phosphor coating 114. The shadows line up with black stripes that separate the red, green and blue vertical phosphor stripes on phosphor coating 114, and because of the fine pitch and regular spacing, the vertical stripes are usually not noticed.
Referring now to FIG. 2, a diagram of the FIG. 1 aperture grill 120 is shown, according to the present invention. Because the metal aperture grill 120 is stretched taut and welded in place over a metal frame, the sheet has a tendency to xe2x80x9cringxe2x80x9d in response to mechanical stimulus, such as mechanical shock or coupled acoustic power. Such ringing causes distracting time-varying luminance modulation as the shadows of the aperture grill 120 beat with the pattern of the phosphor stripes on phosphor coating 114.
A countermeasure employed to minimize the extent of the ringing is the addition of one or more damping wires 210 stretched across the grill 120 assembly, perpendicular to the vertical slits. As a result of friction with the moving aperture grill 120 and internal elastic losses, this wire dissipates mechanical energy to thereby reduce the extent of the ringing. The damping wires 210 cast a shadow on the phosphor-coating 114, but since the damping wire 120 diameter is smaller than that of the visible beams 118 cross section, the shadow is of relatively low contrast. There are typically two damping wires 210 present if CRT 110 is 17 inches or larger and there is typically one damping wire if CRT 110 is smaller than 17 inches. The damping wire shadow is caused by the modulation of the amplitude of the electron beams 118 incident on the phosphor coating 114.
Since there are only one or two damping wire 210 shadows on the screen of CRT 110, and they are perpendicular to the regular fine-pitch structure of the vertical stripes, they are sometimes easily observed, though often not distracting. None the less, when first observed, these shadows are often perceived by as defects in CRT 110. This perception is sometimes a factor in customer satisfaction and overall perception of image and product quality. Therefore, an improved method and apparatus for reducing visibility of damping wires in aperture grill display tubes is needed.
In accordance with the present invention, a method and apparatus are disclosed for reducing visibility of damping wires in aperture grill display tubes. This invention reduces the visibility of damping wire shadows by modulating the amplitude of the CRT electron beam to compensate for the modulation imposed by the damping wires, thus canceling the damping wire shadow. The invention is composed of an apparatus and technique to locate the damping wire shadow with respect to the displayed video image, and an apparatus that generates the compensation signal and applies to the video signal to compensate for the damping wire shadow.
In the preferred embodiment of the present invention, a detector device measures amplitude values of selected areas on a CRT screen as individual CRT scan rows are sequentially illuminated. The measured amplitude values are then processed with a differentiation routine to determine the specific location of the damping wire shadow on the CRT screen.
Correction values to compensate for the damping wire shadow are calculated by subtracting the scan row amplitude value at the damping wire shadow from an average of the scan row amplitude values for the two scan rows adjacent the damping wire shadow. The correction values are then converted into corresponding video values which are stored sequentially and alternately into two display lists along with corresponding location information such as column and row addresses.
A compensator device then sequentially provides the video values and their corresponding location information to video amplifiers to drive the display CRT. The present invention thus advantageously compensates the CRT electron beam current at the appropriate time to effectively remove the damping wire shadows from the aperture grill display tube.